This invention relates to the measurement of particle content in gases, and more particularly to devices and methods for measuring the liquid water content (LWC) in a sample gas such as an atmospheric sample (i.e. clouds or fog).
LWC has been measured with various techniques, three of which will now be described. LWC is defined as the mass of liquid water per unit volume of a sample gas. In the first technique, a thin heated wire is permitted to intercept cloud or fog droplets moving by in an air stream. The heat required to keep the wire at a uniform temperature is approximately proportional to LWC. In the second method, droplets are sized with an optical particle counter and the volume of all droplets per unit atmospheric volume is summed to obtain LWC. In the third method, an optical measurement is made simultaneously on a large number of droplets to deduce LWC. This is done by measuring atmospheric light extinction using a collimated light source with a wavelength near 11 microns P. Chylek has shown that the extinction at this wavelength is proportional to LWC. Reference may be made to an article entitled "Extinction and Liquid Water Content of Fogs and Clouds" in J. Atmos. Sci., Vol. 35. pp. 296-300 for an extensive discussion of Chylek's technique.
Various disadvantages in the above described techniques are typical in prior devices and methods for measuring LWC. For example, the first method has a limited threshold for smaller water droplets (i.e. below 2 microns in diameter). The second and third methods of measuring LWC described above require complex and expensive instrumentation. In particular, the third method of Chylek requires the use of infrared light, and thus requires the use of expensive optical components. In addition, method three relies on light extinction and requires a relatively lengthy device. The size of such a device is critical in many situations, for example in an airplane, where space limitations exist.